Curable mixtures based on glycidyl compounds and amine hardeners are widely used in the industry for coating and improving metallic and mineral surfaces.
The amines used are, in particular, aliphatic, cycloaliphatic, aromatic or araliphatic compounds and, optionally, imidazoline group-containing polyaminoamides based on mono- or polybasic acids as well as adducts thereof.
These compounds are described in Lee and Neville, Handbook of Epoxy Resins, 1967, chapter 6/1 to 10/19.
Some of the curable mixtures based on epoxy resins and on said amine compounds have long curing times.
However, certain applications, especially in the adhesives field, require rapid curing. Acceleration is in many cases also indispensable in the area of the aminoamides/imidazolines, some of which have curing times of several days.
Tertiary amines, acids, hydroxylamines, Mannich bases and phenols are usually used to accelerate these hardeners.
Such accelerators are listed, inter alia, in Lee and Neville, Handbook of Epoxy Resins, 1967, chapter 10, table 10-9.
Bisphenol A (2,2-bis(4-hydroxyphenyl)propane) has been found to be particularly suitable since it additionally improves the early water resistance of the cured thermoset when used as accelerator additive. Bisphenol A is usually added in an amount of 1 to 25%, preferably of 10 to 20%, based on the hardener.
However, disadvantages have been found to be on the one hand the toxicological properties of bisphenol A (endocrinic modulation, oestrogenic effect) and, on the other hand, the high water susceptibility of the formulated hardener. If, to additionally accelerate the resin/hardener system, 1 to 5% of water are added, then the bulk of the dissolved bisphenol A precipitates again in salt form.
The same happens if the hardener absorbs minor amounts of water owing to high atmospheric humidity or for other reasons. Even an absorption of less than 1% of water may, depending on the proportion of dissolved bisphenol A in the hardener, result in the crystallisation of the bisphenol A salt. This is a major disadvantage since the hardener can no longer be processed or must be worked up again at great expenditure.
Accordingly, it is the object of this invention to provide an accelerator which, while having a comparably good accelerating effect and early water resistance, is toxicologically safe and not affected by water being added or adsorbed to the hardener in the range of up to 5%. Surprisingly, it has been found that novolaks of the general formula I are capable of over-coming the above disadvantages also at higher molecular weights and that they are unaffected by addition of water in the range of up to 5 g of water/100 g of hardener formulation while having a comparably good acceleration effect and early water resistance.
This invention therefore relates to curable mixtures based on epoxy resins and amine hardeners, if required additionally using solvents, water, plasticisers, UV stabilisers, colourants, pigments, fillers, which mixtures preferably contain as accelerator 1 to 25% by weight, particularly preferably 10 to 20% by weight, based on the hardener or amine hardener, of at least one compound of the general formula I 
wherein R1, R2, R3, R4 are each independently of one another H or an unbranched or branched alkyl radical containing 1 to 15 carbon atoms, and n is 0 to 10.
In the curable mixtures according to the invention, the novolaks of formula I are used in amounts of 1 to 25% by weight, particularly preferably 10 to 20% by weight, based on the hardener or amine hardener.
Other subject matters of this invention are characterised by the claims.
The novolaks additionally used in accordance with this invention can be prepared by processes known per se by reacting formaldehyde and phenols, such as phenol, cresol, xylenol, alkylphenol and the like, if required additionally using a catalyst, for example oxalic acid, with elimination of water.
Such products are described, inter alia, in Houben-Weyl, 4th edition, Methoden der Organischen Chemie, Vol. E 20, Makromolekulare Stoffe, Part 3, pages 1800-1806.
To prepare these products, the phenol is generally placed in a vessel with addition of waterxe2x80x94sufficient to dissolve phenol and formaldehydexe2x80x94and of catalytic amounts of oxalic acid, subsequently adding formaldehyde, preferably paraformaldehyde, in portions. The volatile components are then removed by distillation under reduced pressure.
The novolaks obtained are statistical mixtures wherein n=0 to 10, preferably n=1 to 3. The range of the average value of n can be determined by using the phenolic component in a suitable excess amount to the amount of formaldehyde used.
The epoxy compounds additionally used according to this invention for the preparation of the curable mixtures are commercially available products containing more than one epoxy group per molecule and are derived from monoxe2x80x94and polyvalent, monoxe2x80x94and/or polynuclear phenols, in particular bisphenols, and from novolaks. An extensive enumeration of these di- and polyphenols is to be found in the compendium xe2x80x9cEpoxidverbindungen and Epoxidharzexe2x80x9d by A. M. Paquin, Springer Verlag, Berlin, 1958, chapter IV, and in Lee and Neville, xe2x80x9cHandbook of Epoxy Resinsxe2x80x9d, 1967, chapter 2, pages 257-307.
It is also possible to use mixtures of two or more epoxy resins. It is preferred to use glycidyl compounds based on bisphenol A having epoxy values of 0.4 to 0.56. Mixtures of the epoxy resins with so-called reactive diluents, e.g. monoglycidyl ethers of phenols or of mono- or difunctional aliphatic or cycloaliphatic alcohols, can also be used. These reactive diluents primarily serve to reduce the viscosity and should only be used in small amounts as they adversely affect the end properties of the thermoset. The epoxy resins mentioned as examples can be used both for the curable mixtures and for the preparation of the hardener-epoxy adducts which may be used.
The hardeners for epoxy resins additionally used according to this invention are aliphatic, cycloaliphabc, araliphatic or aromatic amines, optionally imidazoline group-containing amino-amides and their adducts with glycidyl compounds, which contain on average more than two reactive active hydrogens bound to amino nitrogen atoms per molecule. These compounds are part of the general state of the art and are described, inter alia, in Lee and Neville, xe2x80x9cHandbook of Epoxy Resinsxe2x80x9d, MC Graw Hill Book Company, 1987, chapter 6-1 to 10-19.
Hardeners and epoxy compounds are preferably used in about equivalent amounts, i.e. based on active hydrogens bound to amino nitrogen atoms and reactive epoxy groups. However, it is also possible to use the hardener or glycidyl component in more or in less than the equivalent amount. The amounts used depend on the desired final properties of the reaction product. Standard catalytic hardeners for epoxy resins can be used in addition to the inventive accelerators.
Depending on the field of application and on the end use requirements it is also possible to additionally add to the novel epoxy resin/hardener mixtures inorganic and/or organic additives, such as finely particulate sands, talcum, silicic acid, alumina, metal or metal compounds in chip and powder form, thixotropic agents, fibrous substances (e.g. asbestos and glass-staple fibres), pigments, flame-retarding agents, solvents, colourants, plasticisers, bitumen, mineral oils.